Television receiving system



Sept. 21, 1943. A. H. ROSENTHAL 2,330,171

TELEVISION RECEIVING SYSTEM Filed Jan. 27, 19:59 a Sheets-Sheet 1 g E I g 5 f5 14 M 4Il|| T frzwrzfor fido? fl fosen/thal/ w rg Sept. 21, 1943. 2,330,171

A. H. ROSENTHAL TELEVISION RECEIVING SYSTEM Filed Jan. 27, 1939 3 Sheets-Sheet 2 Sept. 21, 1943. A. H. ROSENTHAL 2,330,171

TELEVISION RECEIVING SYSTEM Filed Jan. 27, 1939 Sheets-Sheet s hmvzz or for/Lew" Patented Sept. 21, 1943 TELEVISION RECEIVING SYSTEM Adolph Henry Rosenthal, New York, N. Y., assignor, by mesne assignments, to Scophony Corporation of America, New York, N. Y., a corporation 01' Delaware Application January 27, 1939, Serial No. 253,182 In Great Britain February 3, 1938 (Cl. Hit-7.5)

33 Claims.

The present invention relates to television receiving systems:

It has been proposed to use in a television receiver an image screen, the opacity or the reflecting power of which changes from point to point according to the intensity values of the received picture signals, so that such a screen if viewed directly or imaged by the light from a separate source on to another screen gives a representation of the picture. For such screens it has been proposed to use mechanical shutters, electro-optical or dichroic media, orientation effects in colloids and like substances, the changes in opacity or reflecting power being usually effected by scanning the screen with a cathode ray beam modulated in intensity in accordance with the received picture signals.

The present invention is concerned with a novel form of screen of this type, and to this end makes use of certain physical effects, discovered and investigated in connection with work on electric conduction in solid bodies, these effects also being closely connected with electrical and thermal changes in luminous phosphors, and with the formation of the photographic image.

If certain crystals, which are normally transparent to visible light, are struck by a beam of cathode rays, X rays, radium rays or by light of a suitable wave length, a deposit of opaque material, which is constituted by what will hereinafter be referred to as opacity centres, is created in this crystal, the degree of opacity depending on the intensity of the incident radiation. Examples of such crystals are many of the alkali and alkaline earth halides, such as the chlorides, bromides and iodides of sodium and potassium, lithium bromide, calcium fluoride, and strontium fluoride, and chloride; and also certain silver salts such as silver bromide. All these crystals belong to the class of the so-called ionic crystals, in which there are electrically positive and negative components, and the forces that hold these components together are electrostatic, at least in part. In the case of the alkali halide crystals research has indicated that the opacity centres probably consist of neutral alkali atoms which are loosely bound in the interior of the crystals in some manner or other, and which are similar to the deposit of metallic silver in a latent photographic image. The deposit of metal in the crystal lattice can also be created by heating an alkali halide crystal in an atmosphere of the vapour of its alkali metal, which diffuses into the crystal.

Once formed, the opaque deposit can also be destroyed by the above mentioned rays, the amount of destruction in a given time interval depending on the intensity of the rays and on the density of the deposit already formed. Thus the gross effect of any given intensity of the incident radiation, being the result of an equilibrium between the formation and destruction of the deposit, may be an increase of the deposit for low intensities and a decrease for the high intensities, in a manner similar to the well known "solarisation" of the latent photographic image.

' Thus, over a range of low intensities of the incident radiation, increase in intensity will result in an increase of the deposit, whilst over a range of high intensities an increase in intensity will result in a decrease of the deposit.

The materials exhibiting this property may be defined as ionic crystals in which the injection of electrons into the crystal lattice can produce an opaque deposit which can be moved within the crystal lattice by application of an electric field and heat.

The present invention contemplates the use of a transparent crystalline material of the type defined in the image screen of a television receiver. The material may be in the form of a single flat crystal, a mosaic of small crystals, or a micro-crystalline structure. A composite crystal or a mixture of two or more of such crystalline materials may be used.

In most cases, and particularly when the material is in the form of a single crystal, a disappearance of the opaque deposit can be produced by maintaining the crystal in an electric field and at a suitable temperature, in which case the deposit is drawn through the crystal towards the positive pole producing the electric field. When it reaches the positive pole it disappears, leaving the crystal substantially transparent. The speed of movement of the deposit depends upon the strength of the field and upon the temperature, and can be varied within wide limits by varying either magnitude. For a given field strength this speed of movement increases with the temperature of the crystal.

According to the present invention there is provided a method of television reception which comprises scanning an image screen including a material of the type described with a beam of radiant energy modulated in intensity in accordance with the received picture signals to produce periodically at frame scanning frequency in each elemental area of said screen an opaque deposit the density of which diflers from a fixed datum level of density by an amount depending upon the instantaneous value of the intensity of the beam striking the area, and causing the density of said deposit to return to said datum level, the frequency of the return or the successive deposits in said elemental axea being equal to the frame scanning frequency.

The datum level of density may be zero, in which case the scanning beam produces directly an opaque deposit in each elemental area of the image screen proportional in density to the instantaneous intensity of the beam when it strikes the area, and this deposit is caused to disappear periodically at the frame scanning frequency.

Alternatively the datum level of density may correspond to picture black. in which case the scanning beam is adapted to remove the deposit from each elemental area to an extent depending upon the instantaneous intensity of the beam when it strikes the area, and the density of the deposit in each elemental area is caused to returned to a maximum value corresponding to picture black periodically at frame scanning frequency.

The invention will now be described by way of example with reference to the accompanying drawings in which:

Figs. 1 and 8 show forms of the apparatus used according to the invention in which the image screen consists of a single crystal scanned by a cathode ray beam:

Fig. 3 shows the use of a light beam for scanning the crystal;

Figs. 4 and 5 show apparatus for heating the image screen; and

Figs. 6 and 7 show schematically an alternative method of carrying the invention into effect.

Referring to Fig. l, a cathode ray tube l is provided with a cathode 2, a control grid 3, a beam focussing coil 4, deflecting coils 5, G, and an accelerating anode 1. Picture signal. from the receiver 8 are applied between the cathode and control grid in such a way that the positive potential of the grid decreases with increase in signal strength, so that a modulated beam is produced and is swept over the image screen in the usual manner. The image screen consists of a flat crystal 9 of an alkali halide such as potassium chloride, provided on each side with an electrode I0, I I designed to permit the passage of light. These electrodes are shown in the form of thin transparent spattered metallic layers, but they can also be in the form of fine meshes or the like. The potential of the electrode H is maintained positive with respect to that of the electrode It to provide an electric field in the crystal. The crystal 9 is traversed by light from the incandescent lamp I! which is concentrated on the projection lens l5 by the optical condenser l3, and an image of the crystal is formed on th jection screen H by means of the projection lens 15.

The apparatus operates as follows:

On striking a given elemental area of the crystal 9. the modulated cathode ray beam produces therein an opaque deposit of a density proportional to the instantaneous intensity of the beam. After the beam leaves this area the deposit persists and moves through the crystal in the direction of its thickness towards the more positive electrode H, where it disappears. This phenomenon can be explained by assuming that the incident cathode ray beam injects into the elemental area of the crystal a-number of electrons corresponding to the instantaneous intensity of the beam when it strikes the area. These tend to travel as free electrons towards the positive electrode between the crystal lattice, which is com posed of alternate positive alkali ions and negative halogen ions. During this travel certain electrons will be captured by the alkali ions, which have a great electron ailinlty. An alkali ion and an electron together form an electrically neutral metallic alkali atom which constitutes the above mentioned opacity centre, and thus the position of each captured electron is made visible in the form of an opacity centre. The impinging electrons of the cathode ray beam may release secondary electrons in greater numbers on their impact. These secondary electrons also tend to travel inside the crystal lattice, thus increasing the effect. Sometime later, by the heat movement of the lattice (the crystal being held at the necessary temperature) the metallic alkali atom is again split up into anion and an electron, and the freed electron continues its path through the lattice towards the picture electrode until it is again captured by another alkali ion, forming a visible opacity centre nearer to the positive electrode. Thus the stream of electrons shot into the crystal by the beam and moving towards the positive electrode appears in the form of an opaque deposit constituted by the opacity centres and moving through the crystal towards the positive electrode and disappearing there.

The velocity of this opaque deposit is proportional to the electric field strength in the crystal and increases also with an increase in temperature of the crystal. By a suitable choice of these magnitudes in relation to the thickness of the crystal it can be arranged that the deposit of a given elemental area traverses the thickness of the crystal in substantially the picture frame scanning period, i. e. during the time interval between two consecutive scannings of the elemental area by the beam. In other words, the frequency of the disappearance of the successive deposits is caused to be equal to the frame scanning frequency. The opacity of a given elemental area, which corresponds to the intensity of the beam when it strikes the area will thus remain constant until the beam strikes the area at the next scan, when it will immediately adjust itself to the new value. Thus each elemental area of the image screen maintains its intensity value constant for the whole duration of a picture frame, as distinct from known arrangements in which the intensity values decreasesuddenly, or in the most favourable cases, exponentially or hyperbolically during the frame period.

Although the deposits produced in a given elemental area must be caused to disappear periodically at substantially frame scanning frequency, the disappearance of one deposit need not coincide exactly with the formation of a new deposit in that area but can occur at slightly later time. This can be achieved by regulating the velocity '"of the opaque deposit produced in an elemental area by one scan in such a way that it has not quite reached the positive electrode when the succeeding scan reaches the area. Thus any desired slight overlapping may be obtained.

From the foregoing it is obvious that the picture repetition frequency can be much lower than is usual with normal reception methods since the intensity is held constant during the whole frame period and no flickering occurs. The minimum repetition frequency is now determined only by the demands of the eye in perceiving continuous movement, and can be about 17-20 frames per second. This enables a. considerable'reduction members 20, 2|.

in the necessary Irequency band width of the transmitted signals to be achieved. or allows with the same band width a, higher definition to he obtained, or permits of the use of the free part of the band for other purposes.

The fugitive image produced on the crystal screen can be regarded as the equivalent of a photographic image on a lantern slide or a cinema film. But this pseudo photographic image lasts only for the duration of a frame scanning period, and is then replaced by the new image on the same carrier. Thus the effect is similar to that of the so-called intermediate film process, but with the advantages that (1) no time is lost for processing, and (2) one frame is replaced by the next on the same carrier, no film being consumed. The interchange of the consecutive frames is brought about by the diilusion of electrons across the thicknes of the crystal under the influence of the electric field.

It is not essential to have two electrodes as shown for setting up the electric field. The electrode II) can usually be dispensed with since the cathode ray beam striking the surface of the crystal 9 will cause an emission of secondary electrons, thereby setting up an equilibrium potential of a certain fixed value. The electrode I I is then maintained positive with respect to this equilibrium potential. In certain cases the electrode II can also be dispensed with. For example if the ratio of secondary electrons ejected from the crystal to primary electrons incident on the crystal is less than 1, the equilibrium potential will approach that of the cathode 2. in which case the potential of the opposite surface of the crystal will be more positive to an extent depending on the leakage resistance between the anode l and the end wall of the tube I. This leakage resistance may be predetermined by giving to the inner surface of the tube a certain conductivity, in any known manner.

The surface struck by the cathode ray beam may be provided with a secondary electron emitting layer for which the ratio of secondary electrons emitted thereby to primary electrons incident thereon is high, such as beryllium. By this means, full use of the increase in the effect caused by secondary electron emission previously mentioned may be made.

In Fig. 2 is shown an alternative arrangement in which the crystal 9 is situated outside the cathode ray tube I and inside a container '1 which forms a continuation of the tube. The end wall of the tube comprises a thin layer of metal foil II; which permits of the passage of the cathode ray beam, and which also serves as one electrode of the crystal. In this case the image of the crystal is formed on the projection screen l4 with the aid of light reflected or scattered from the layer It as shown. Alternatively, the crystal can be illuminated with diffuse light and viewed directly, thus presenting a surface image similar to a photographic paper image.

In Fig. 3 is shown the use of a light beam instead of a cathode ray beam for creating the opacity in the crystal. A beam of light from the source 18, shown as a luminous discharge tube is modulated in intensity in accordance with received picture signals by the light modulator I9, which can be of any suitable type, and is caused to scan the crystal by means of the two scanning Light from the source I2, is shown as an incandescent lamp, is utilized to form an image of the crystal on the projection screen l4. The maximum emission of the light source l8 must occur at waveleng app p t the production of the necessary opacity in the crystal, which are usually in the region of the shorter wavelengths, whilst the maximum emission of the light source I2 must occur at different wavelengths in order that it should play no part in the formation of the opacity in the crystal. This differentiation may be assisted, if desired by the use of suitable filters, indicated at 22 and 23.

In the embodiment described, no special means have been shown for maintaining the crystal at a constant predetermined temperature. In many cases the heat produced by the incident cathode ray beam, or by the heat rays emitted by the incandescent lamp II, or by both, will be found suilicient to maintain the crystal at the desired temperature. Where higher temperatures, or a more exact temperature control is required, special means may be provided, two examples of which are illustrated in Figs. 4 and 5.

In Fig, 4 the crystal 9 is heated by means of an electric current from a source 24 which is passed through the electrode l I via the conductors 25, 26. This heating current is controlled to maintain the crystal at a constant predetermined temperature by means of the thermocouple 21 in serted into the crystal and electrically connected to control the source 24 in any suitable manner. This heating can be applied to any of the electrodes shown in the previous figures.

In Fig. 5 the heating is effected by means ofan oven 28 surrounding the crystal 9 and comprising a heating coil 29 fed with current from the source 30, the heating current being controlled by means of the thermocouple 21. The oven is provided with a window 3| to allow the necessary light to illuminate the crystal. In the arrangement of Fig. 2 a flow of air or other gas heated to a controlled temperature can be circulated through the container I1 to heat the crystal.

In the arrangement hitherto described the removal of the opaque deposit has been caused by maintaining the image screen in an electric field and at a suitable temperature so as to produce a mobility of the deposit. However, as already mentioned, the application of intense cathode rays or light of a suitable wavelength to the material also produces a removal of the deposit. This method may be used instead of or in addition to the method hitherto described particularly in cases where a sufllcient mobility of the deposit is difiicult to obtain. The method will be described with reference to Figs. 6 and 7.

In Fig. 6 there is shown a cathode ray tube I, in plan view, which is provided with an image screen consisting of a micro-crystalline deposit of a suitable alkali halide such as potassium chloride, sodium iodide or a mixture of the two. The tube is provided with two electron gun structures provided with a common focussing coil 4, and common deflecting coils 5, 6 and a common mode 44. One structure includes a cathode 4| and a control grid 42, connected across a source of picture signals 43. This structure produces a cathode ray beam of an intensity suitable to the production of an opaque deposit in the material of the image screen 40, and the intensity of the beam is modulated in accordance with the received picture signals in such a way that the intensity of the beam decreases with increase of signal strength. The second structure includes a cathode 46 and a grid 41, the latter being maintained at such a positive potential with respect to the cathode 46, that an unmodulated cathode ray beam I of high intensity is produced, this intensity being suillcient to cause a removal of the opaque deposit produced by the beam 45. The relative positions of the two scanning spots produced by the two beams are shown in Fig. 7 where the line scanning is effected in the direction of the arrow 48 being the picture modulated image forming spot corresponding to the beam 45, whilst 50 is the deposit removing spot corresponding to the beam M and which precedes I! and is closely adjacent thereto. It

will be observed that the deposit in an elemental area of the screen 40 produced by the spot 49 will persist for a frame scanning period, and will then be removed by the spot 50 and immediateiy replaced by the new deposit produced by the spot IS.

The apparatus illustrated in Fig. 6 may be operated in an alternative manner, by reducing the positive bias on the grid 41' to such an extent that the beam II is of a low intensity appropriate to the production in the material of the image screen ll of a uniform opaque deposit corresponding to picture black. A large positive bias is then applied to the grid 42 such that the beam 45 is now of high intensity and will cause a removal of this deposit. This beam is modulated in intensity in accordance with the received picture signals in such a way that an increase in intensity of the beam corresponds to an increase in signal strength, so that the deposit will be removed to an extent depending upon the intensity of the beam 45. Thus the deposit remaining in an elemental area of the image screen 46 after the passage of the spot I! will persist for a frame scanning period until the arrival ofthe spot ill, when it will momentarily increase to a value corresponding to picture black, and will again be reduced to its new value by the spot 49.

In most television transmission systems, synchronising signals are transmitted in the intervals between successive lines and frames. In these intervals a fly-back of the cathode ray beam occurs. These synchronising signals are usually of the "blacker than black" type, so that if they are applied to the control grid of a cathode ray tube together with the picture signals, they suppress the cathode ray beam during the fly-back. In cases where a positive control is used, that is, where an increase in the intensity of the cathode ray beam produces an increase in the white" value of the image screen, this method is useful. This case occurs in the ordinary fluorescent screen type of cathode ray tube, and

in the embodiment of the present invention in which the datum level of density of the deposit corresponds to picture black. Where a negative control is used, as in the embodiments of the present invention in which the image screen is initially transparent and the density of the deposit increases with the increase in the intensity of the scanning beam, the blacker-than-black impulse would produce a beam of increased intensity, with the result that a series of opaque lines would be traced by the cathode ray beam during the fly back between successlvepicture lines, an opaque line across the picture would be traced during the fly back between successive picture frames. This disadvantage can be avoided by reversing the direction of the synchronislng signal, applied to the control grid so that they lie in the "white direction and produce a reductlon in the intensity of the beam.

The micro-crystalline layer mentioned in connection with Fig. 6 is very easily obtainable in any desired thickness by subllming the material directly on to the wall of the cathode ray tube, or on to a transparent metallic film which can serve as an electrode. The material is preferably placed in a small boat or container inside the cathode ray tube and connected in series with a ring shaped copper strip and the layer is formed by heating the boat with eddy currents induced in the strip by means of an external induction coil of an eddy current heater. Very uniform deposits of the material or a mixture of materials can be obtained in this manner.

The sensitivity of the crystalline material will depend on the treatment during its formation. Any treatment which tends to disturb the regularity of the lattice will tend to give an increased sensitivity. For example, a rapid formation of the crystal, or the introduction of foreign atoms or molecules into the crystal lattice will usually increase the sensitivity.

Thus in the case of potassium halides, molecules of KB or K20 can be introduced by heating the crystals in an atmosphere of hydrogen or oxygen. Also traces of heavy metals, such as silver or thallium may be added.

In the following claims elemental areas of the ionic crystal material layer are referred to as elemental portions of the layer. Also, in certain of the claims, the deposits created in a normally transparent ionic crystal material layer, or transparent areas in a normally opaque or picture black layer, ,are referred to as areas" having light transmitting qualities differing from the normal light transmitting qualities of the layer.

I claim:

1. In cathode ray tube apparatus for reconstituting pictures from received picture signals comprising an image screenadapted to have its light transmitting faculty varied periodically point by point by a cathode ray beam scanning it, means for developing a cathode ray beam, means for modulating the intensity of said beam and means for deflecting said beam periodically to cause it to scan said image screen, a screen structure constituting said image screen and including an ionic crystal material of the type in which the injection of electrons into the crystal lattice can produce an opaque deposit which can be moved within the crystal lattice by application of an electric field and heat, means for maintaining the surface of said screen which is struck by said beam at a lower positive potential than the opposite surface of said screen, means for heating said screen, and thermostatic controlling means adapted to act on said heating means to maintain constant the temperature of said screen.

2. A cathode ray tube for reconstituting a pictui'e from received picture signals comprising an image screen including an ionic crystal material of the type in which the injection of electrons into the crytsal lattice can produce an opaque deposit which can be moved within the crystal lattice by application of an electric field and heat, means for developing a first beam of cathode rays of low intensity adapted to create an opaque deposit in said material, means for de- Veloping a second beam of cathode rays of a high intensity adapted to reduce said deposit, means for modulating the intensity of one of said beams in accordance with said signals, means for directing said beams onto adjacent elemental portions of said screen so that the unmodulated beam precedes the modulated beam in the line scanning direction, and means for deflecting said beams in synchronism over said screen.

3. In and for electric picture reconstituting apparatus of the scanning transparency control type, a picture screen adapted for use as the transparency-controlled member which includes an ionic crystal material of the type in which the injection of electrons into the crystal lattice can produce an opaque deposit which can be moved within the crystal lattice by application of electric fields and heat, means for scanning said screen with a first beam of radiant energy, means for modulating said beam, in accordance with received picture signals within the range of those low intensities for which the density of the opaque deposit produced in any scanned elemental portion of said image screen differs, by an amount depending upon the instantaneous value of the intensity of said beam when striking said portion, from a datum level represented substantially by zero density, and means for scanning said screen with a second beam of radiant energy having a constant high intensity adapted to remove the deposits formed through the scanning action of said first beam, said last mentioned means being adapted to cause said second beam to precede immediately said first beam in the line scanning direction.

4. In and for electric picture reconstituting apparatus of the scanning transparency control type, a picture screen adapted for use as the transparency-controlled member which includes an ionic crystal material of the type in which the injection of electrons into the crystal lattice can produce an opaque deposit which can be moved within the crystal lattice by application of an electric field and heat, means for scanning said screen with a first beam of radiant energy, means for modulating said beam, in accordance with received picture signals, within the range of those high intensities for which the density of the opaque deposit produced in any scanned elemental portion of said image screen differs, by an amount depending upon the instantaneous value of the intensity of said beam when striking said portion, from a datum level represented by a maximum corresponding to picture black, and means for scanning said screen with a second beam of radiant energy having a constant low intensity adapted to create a uniform deposit in the image screen, said last mentioned means being adapted to cause said second beam to precede immediately said first beam in the line scanning direction.

5. In cathode ray tube picture reconstituting apparatus of the scanning transparency control type, a picture screen adapted for use as the transparency-controlled member which includes an ionic crystal material of the type in which the injection of electrons into the crystal lattice can produce an opaque deposit which can be moved within the crystal lattice by application of an electric field and heat, means for modulating, in accordance with received picture signals the scanning cathode ray beam within the range of those low intensities for which the density of the opaque deposit produced in any scanned elemental portion of said image screen differs, by an amount depending on the instantaneous value of the beam striking said portion, from a datum level represented substantially by zero density, means for maintaining the surface of said screen which is struck by said beam at a lower positive potential than the opposite surface of said screen and means for heating said screen.

6. The combination in an image forming apparatus, of a layer of an ionic crystal material including two opposite faces and mounted to present one of said faces as an image face, means to scan one of said faces with a radiant beam modulated in intensity in accordance with a received signal to vary the light transmitting qualities of successive elemental portions of said layer from a normal datum level by creation in the layer of areas movable between said faces and having light transmitting qualities differing from said datum level to thereby create an image perceptible at said image face and in the direction of movement of said areas, and means to restore the light transmitting qualities of elemental portions of said layer to said normal datum level within a predetermined time period.

7. The combination as set forth in claim 6 wherein said restoring means is a second radiant beam.

8. The combination as set forth in claim 6 wherein said restoring means at least includes means to maintain one of said faces at a higher positive potential than the other.

9. The combination as set forth in claim 6 wherein said restoring means is effective within a time period substantially corresponding to a frame scanning interval.

10. The combination in an image forming apparatus, of a layer of an ionic crystal material including two opposite faces and mounted to present one of said faces as an image face, means to scan one of said faces with a radiant beam modulated in intensity in accordance with a received signal to vary the light transmitting qualities of successive elemental portions of said layer from a normal datum level by creation in the layer of areas movable between said faces and having light transmitting qualities differing from said datum level to thereby create an image perceptible at said image face and in the direction of movement of said areas, and means to restore the light transmitting qualities of elemental portions of said layer to said normal datum level by moving said areas from one of said opposite faces to the other to thereby disappear within a predetermined time period.

11. The combination as set forth in claim 10 wherein said radiant beam is a cathode ray beam.

12. The combination as set forth in claim 10 wherein said layer of an ionic crystal material is in micro-crystalline form.

13. The combination as set forth in claim 10 wherein said layer of an ionic crystal material is an alkali halide.

14. The combination as set forth in claim 10 wherein said layer of an ionic crystal material is an alkali halide in micro-crystalline form.

15. The combination as set forth in claim 10 wherein said layer of an ionic crystal material includes on its scanned face a coating having a high degree of secondary electron emission.

16. The combination as set forth in claim 10 wherein said means to restore the normal light transmitting qualities of an elemental portion includes at least means to maintain said scanned face at a lower positive potential than the opposite face.

17. The combination as set forth in claim 10 wherein said means to restore the normal light transmitting qualities of an elemental portion im cludes at least means to maintain said scanned face at a lower positive potential than the opposite face which comprises a pair of electrodes,

each adjacent one of said faces, at least one of the electrodes being transparent.

18. The combination as set forth in claim 10 wherein said means to restore the normal light transmitting qualities of an elemental portion includes at least means to maintain said scanned face at a lower positive potential than the opposite face which comprises a pair of transparent electrodes, each positioned at and coextensive with one of said opposite faces.

19. The combination as set forth in claim 10 wherein said means to restore the light transmitting qualities of an elemental portion includes at least means to maintain said scanned face at a lower positive potential than the opposite face which comprises a pair of electrodes, each adjacent one of said faces, one of the electrodes being light reflecting and the other being transparent.

20. The combination as set forth in claim 10 wherein said means to restore the normal light transmitting qualities of an elemental portion includes at least means to heat said material.

21. The combination as set forth in claim 10 wherein said layer of ionic crystal material is transparent at said normal datum level.

22. The combination as set forth in claim 10 wherein said layer of ionic crystal material is at least substantially opaque at said normal datum level.

23. The combination in an image forming apparatus, of a layer of an ionic crystal material including two opposite faces and mounted to present one of said faces as an image face, means to scan one of said faces with a radiant beam modulated in intensity in accordance with a received signal to vary the light transmitting qualities of successive elemental portions of said layer from a normal datum level by creation in the layer of areas movable between said faces and having light transmitting qualities differing from said datum level, means to restore the light transmitting qualities of elemental portions of said layer to said normal datum level within a predetermined time period, and means to illuminate at least one of said opposite faces of said layer with light of a wave length which will not affect the light transmitting qualities of said layer and from such direction that changes in light transmission will be perceptible at said image face.

24. The combination in an image forming apparatus. of a layer of an ionic crystal material including two opposite faces, means to scan one of said faces with a radiant beam modulated in intensity in accordance with a received signal to vary the light transmitting qualities of successive elemental portions of said layer from a normal datum level by creation in the layer of areas movable between said faces and having light transmitting qualities differing from said datum level, means to illuminate said layer with a heat producing source of light of a wave length which will not affect the light transmitting qualities of said layer and from such direction that changes in light transmission can be observed in the direction of movement of said areas, and means to restore the light transmitting qualities of elemental portions of said layer to said normal datum level within a predetermined time period, said restoring means including at least the heat of said illuminating means,

25. An image forming apparatus comprising a cathode ray tube including a layer of a. normally transparent ionic crystal material including two opposite faces and mounted to present one of said faces as an image face, means to scan one of said faces with a cathode ray beam modulated in intensity in accordance with a received signal to create opacity centers at one of said opposite faces of the layer to thereby create an image perceptible in said layer in a direction substantially normal to said faces. means to move said centers from the face at which they are created to the opposite face to thereby disappear within a predetermined time period, and means to direct visible light through the portions of said layer which are free of opacity centers.

26. An image forming apparatus comprising a cathode ray tube including a layer of a normally transparent alkali halide material including two opposite faces and mounted to present one of said faces as an image face, means to scan one of said faces with a radiant beam modulated in intensity in accordance with a. received signal to create opacity centers at one of said opposite faces to thereby create an image perceptible in said layer in a direction substantially normal to said faces, means to move said centers from the face at which they are created to the opposite face to thereby disappear within a predetermined time period, and means to direct visible light upon one of said faces.

27. An image forming apparatus comprising a cathode ray tube including a layer of 9, normally transparent alkali halide material including two opposite faces and mounted to present one of said faces as an image face, means to scan one of said faces with a radiant beam modulated in intensity in accordance with a received signal to create opacity centers at one of said faces of the layer to thereby create an image perceptible in said layer in a direction substantially normal to said faces, means to move said centers from the face at which they are created to the opposite face to thereby disappear within a predetermined time period, and means to direct visible light upon one of said faces and thence to a viewing screen.

28. An image forming apparatus comprising a cathode ray tube including a. layer of a normally transparent alkali halide material including two opposite faces and mounted to present one of said faces as an image face, means to scan one of said faces with a cathode ray beam modulated in intensity in accordance with a received signal to create opacity centers at one of said faces of the layer to thereby create an image perceptible in said layer in a direction substantially normal to said opposite faces, means to maintain the face at which the centers are created at a lower positive potential than the opposite face to move said centers from the face at which they are created to the opposite face to thereby disappear within a predetermined time period, and means to direct visible light through at least the portions of said layer which are free of opacity centers.

29. The combination in an image forming apparatus of a layer of an alkali halide material including two opposite fees and mounted to present one of said faces as an image face, means to scan one of said faces with a first radiant beam modulated in intensity in accordance with a received signal to vary the light transmitting qualities of successive elemental portions of said layer from a normal datum level by creation in the layer of areas movable between said faces and having light transmitting qualities differing from said datum level to thereby create an image perceptible at said image face and in the directlon of movement of said areas, and means to scan one of said faces with a second beam of constant intensity to restore the light transmitting qualities 0! said layer to the normal datum level.

30. The combination as set forth in claim 6 wherein said layer of an ionic crystal material is in micro-crystalline form 31. The combination as set forth in claim 6 wherein said layer of an ionic crystal material 10 is an alkali halide.

ADOLPH HENRY ROSENI'HAL. 

